1. Field of the Invention
The present invention relates to a semiconductor device having a chip-on-chip structure having semiconductor chips stacked, on their surface, with other semiconductor chips.
2. Description of Related Art
There is a multi-chip module that is one form of semiconductor devices intended to improve the transmission speed of the signals at between devices. In the multi-chip module, a plurality of semiconductor chips are mounted on a circuit board in one package. By shortening the wiring connecting between the semiconductor chips, high speed transmission of a signal is achieved. A plurality of semiconductor chips forming functional elements are mounted on the circuit board, wherein each semiconductor chip is connected face down on the circuit board (flip-chip connection). The circuit board, usually, uses an insulating board formed with multiple levels of wiring thereon. Namely, the circuit board includes the wiring on a surface layer, the wiring on inner levels, and the wiring of inter-level connection.
The semiconductor chip mounted on the circuit board in a certain case is further stacked with other semiconductor chips thereby forming a chip-on-chip structure.
In the multi-chip module thus structured, by shortening the length of the wiring connecting between the circuit board and the semiconductor chip and between the semiconductor chips in the circuit board, it is hoped that high speed transmission of signals between the functional elements formed on the semiconductor chips will be realized. Also, because connection is possible to another circuit board through the bumps provided at the underside of the circuit board, the wiring length for external connection is comparatively short thus making it possible to externally transmit a signal with a certain degree of speed.
However, in the multi-chip module, the wirings on the circuit board are greater in width and pitch as compared to the wiring made by the semiconductor process. Consequently, in order to mount semiconductor chips with high density on the circuit board, there is a need to increase the number of levels of wirings in order for preventing against interference between the wirings, resulting in an increased wiring length. This makes it impossible to sufficiently increase the transmission speed of signals.
Meanwhile, the semiconductor chips are externally connected through the circuit board using an insulating substrate and hence the possible wiring-length reduction is limited. This makes it impossible to sufficiently increase the signal transmission speed over the external connection.
It is an object of the present invention to provide a semiconductor device capable of increasing the transmission speed of the signal.
A semiconductor device of the present invention is a semiconductor device structured by mutually connecting a first semiconductor chip to second and third semiconductor chips arranged side by side, with their active surfaces faced to each other. The second and third semiconductor chips respectively have functional elements in their active surface. The first semiconductor chip is wired on its active surface to the second and third semiconductor chips, and a terminal for external connection on the surface opposite to the active surface.
Because the wiring of the first semiconductor chip is formed by a semiconductor process, wire width and interval between wires can be made 1 xcexcm or smaller, for example. On a circuit board using an insulating substrate, wire width and interval between wires is several tens to several hundreds of xcexcm. As compared to this, the wiring by the semiconductor process is more precise by far. Due to this, the wiring on the first semiconductor chip, even in multi-levels, can be made in a short length. Accordingly, the second semiconductor chip and the third semiconductor chip can be connected (internally through the first semiconductor chip) together with a short length of wire.
Meanwhile, the first semiconductor chip has a terminal for external connection on the surface opposite to the active surface. The terminal for external connection can be made by a bump formed by a solder ball or the like. Using the terminal for external connection, surface mounting can be made onto another circuit board. Accordingly, because the semiconductor chips can be externally connected through a short distance without interposing a circuit board, the semiconductor device has a high external signal transmission speed.
As described above, the semiconductor device structured as above can have a short wiring length in both internal and external connections, hence making it possible to increase the transmission speed of the signal.
The first semiconductor chip may be connected with another semiconductor chip besides the second and third semiconductor chips. Also, the second or third semiconductor chip may be vertically stacked and connected thereon with another semiconductor chips.
By structuring the first to third semiconductor chips of the same kind of semiconductor material (e.g. silicon), these can be made coincident in their thermal expansion coefficients. Thus, it is possible to avoid the stresses from occurring due to a difference in thermal expansion/contraction.
The first semiconductor chip may have a functional element on its active surface.
By providing the first semiconductor chip with a function in addition to its role as a circuit board, it is possible to provide a dense arrangement of the semiconductor chips having functional elements. This realizes size reduction and functional improvement of the semiconductor device.
Meanwhile, by providing a functional element on the first semiconductor chip, the functional elements are rendered in a state arranged uniformly in the first to third semiconductor chips, thus shortening the mean wiring length. Namely, the first semiconductor chip is provided with wiring connecting the functional elements of the first semiconductor chip and connecting the functional elements of the first semiconductor chip and the functional elements of the second or third semiconductor chip. Most of the wirings are short as compared to the wiring connecting the functional elements of the second and third semiconductor chips.
This makes short the overall wiring length as compared to the case connecting the functional elements of the semiconductor chips arranged side by side as in the conventional multi-chip module. This further improves signal transmission speed.
The first semiconductor chip may have a through-hole.
The through-hole provided in the first semiconductor chip can be provided, therein, with a conductor. In this case, short distance connection is possible between the wiring on the active surface and the terminal for external connection through the conductor within the through-hole. This structure further increases signal transmission speed to and from the outside.
The through-hole, at its inside, may be filled with a conductor by using a conductor paste. In such a case, a terminal for external connection may be provided immediately beneath the through-hole. This makes minimal the distance of between the wiring on the active surface and the terminal for external connection (nearly equal to a thickness of the first semiconductor chip). There is no need to connect all the wirings on the active surface of the first semiconductor chip to the terminal for external connection through the through-hole. It is possible to selectively connect the wiring required for high-speed signal transmission by the through-hole to the outside.
The second and third semiconductor chips each may have a wiring connecting internal functional elements. In this case, the wires in the first semiconductor chip are preferably greater in sectional area than the wiring on the third semiconductor chip.
The wiring of the first semiconductor chip, formed by the semiconductor process, is narrow in width. The wires, when made small in sectional area, have an increased resistance per unit area, thus increasing the overall wiring resistance. According to the above structure, the wiring of the first semiconductor chip is greater in sectional area than the wiring of the second or third semiconductor chip. Accordingly, because the wiring of the first semiconductor chip is low in resistance per unit length, the overall wiring resistance is low in the first semiconductor chip. This structure can provide the first semiconductor chip with excellent function as a circuit board. There is no need to provide the same sectional area to all the wires in the first semiconductor chip, e.g. the longer wiring may be selectively increased in its sectional area.
In order to increase the sectional area of the wiring of the first semiconductor chip greater than the wiring of the second or third semiconductor chip, it is possible to increase the width or thickness thereof.
The above and other objects, features and effects of the present invention will be more apparent from the ensuing embodiment to be explained referring to the accompanying drawings.